Exhaust emission control device and casing structure of the control device

ABSTRACT

An exhaust gas purifying device includes a tubular casing arranged in exhaust passages of an internal combustion engine. A filter is held in the casing. The filter collects and burns particulates contained in the exhaust gas discharged by the internal combustion engine. The casing has a double structure including an inner case supporting an outer peripheral surface of the filter and an outer case arranged around the inner case. The inner and outer cases are spaced from each other with a clearance defined between the cases.

FIELD OF THE INVENTION

The present invention relates to exhaust gas purifying devices forpurifying exhaust gas discharged by internal combustion enginesincluding diesel engines and casing structures for the devices.

BACKGROUND OF THE INVENTION

Conventionally, a number of exhaust gas purifying devices have beenproposed. The devices are configured by accommodating a ceramichoneycomb filter for purifying exhaust gas in a metal tubular casing,which is disposed in an exhaust passage of a diesel engine. As thefilter is used for a relatively long time, the filter collects soot(diesel particulates) from the exhaust gas. The soot is deposited in thefilter and gradually increases the engine load. If this is the case, thefilter is heated to the ignition temperature of the soot (600 to 630degrees Celsius) by a heating means such as a heater or burner. The sootis thus burned and removed such that the filter is regenerated.

However, conventional exhaust gas purifying devices have the followingproblems.

That is, when the filter is heated by the heating means, the heat istransmitted to a different component (for example, the casing) that isheld in contact with the filter. The heat thus escapes to the exteriorof the filter, hampering the heating of the filter. This increases theenergy needed to heat the filter to the soot ignition temperature andraises costs. Further, if an electric heater is used as the heatingmeans, an increased electric load is applied to the battery, whichaccelerates the battery consumption.

In addition, a temperature difference is caused between the middleportion of the filter and an outer peripheral portion of the filter.Therefore, if the honeycomb filter is formed of, for example, poroussilicon carbide, an increased thermal stress is generated in the filter.In this case, the filter has a tendency to crack, which is damaging tothe filter. Moreover, in conventional devices, replacement of the filteris complicated, making it difficult to maintain the devices.

The present invention is for solving the above problems. Accordingly, itis an objective of the invention to provide an exhaust gas purifyingdevice that saves costs by decreasing energy loss and suppressing damageto the filter, as well as a casing structure for the device.

DISCLOSURE OF THE INVENTION

To solve the above problems, the gist of a first embodiment of thepresent invention is an exhaust gas purifying device comprising atubular casing disposed in an exhaust passage of an internal combustionengine and a filter accommodated in the casing for collecting, burning,and removing particulates contained in the exhaust gas discharged by theinternal combustion engine. The device is characterized in that thecasing has a multiple case structure including an inner case supportingan outer peripheral surface of the filter and at least one outer casearranged around the inner case, and that the inner and outer cases arespaced from each other with a clearance defined between the cases.

The gist of a second embodiment is an exhaust gas purifying devicecomprising a tubular casing disposed in an exhaust passage of aninternal combustion engine and a filter accommodated in the casing forcollecting, burning, and removing particulates contained in the exhaustgas discharged by the internal combustion engine. The device ischaracterized in that the casing has a double structure including aninner case supporting an outer peripheral surface of the filter and anouter case arranged around the inner case, and that the inner and outercases are spaced from each other with a clearance defined between thecases.

It is desirable that a fluid blocking member is provided at an upstreamend portion of the inner case for blocking communication between a spaceincluding an upstream end surface of the filter and the clearance.

A fluid blocking member may be provided at a downstream side of theinner case and between the inner case and the outer case for blockingcommunication between a space including a downstream end surface of thefilter and the clearance.

The fluid blocking member may be a flange projecting from an outerperipheral surface of an end of the inner case. The flange may besecured to an outermost component of the outer case in an attachable ordetachable manner.

A heater for regenerating the filter may be disposed at an upstreamposition with respect to the filter. Further, a porous heat reflectormay be arranged at a further upstream position with respect to theheater.

The gist of a third embodiment is a casing structure of an exhaust gaspurifying device having a multiple structure including an inner casesupporting an outer peripheral surface of a filter for collecting,burning, and removing particulates contained in the exhaust gasdischarged by an internal combustion engine and at least one outer casearranged around the inner case. The casing structure is characterized inthat the inner and outer cases are spaced from each other with aclearance defined between the cases.

In each of the above-described embodiments, the outer peripheral surfaceof the filter is held in contact with the inner case. However, theclearance is defined between the inner case and the outer case.Therefore, in other words, a heat insulating air layer is ensuredbetween the inner and outer cases. The heat transmission from the innercase to the outer case is thus hampered. This prevents the heat fromescaping to the exterior of the filter such that the temperature of thefilter is efficiently raised. That is, the exhaust gas purifying devicereduces energy loss and thus saves costs. Further, since the heat isprevented from escaping from the outer peripheral portion of the filter,the temperature difference between the middle portion and the outerperipheral portion of the filter hardly occurs. As a result, arelatively large thermal stress, which leads to cracks damaging thefilter, is avoided.

If the fluid blocking member is provided at the upstream end portion ofthe inner case, communication between the space including the upstreamend surface of the filter and the clearance is blocked. Therefore, arelatively hot exhaust gas does not flow into the clearance in which theheat insulating air layer is formed. This structure reduces heat energyloss caused by heat transmission from the exhaust gas to the outer case.The costs are thus further saved. Further, the exhaust gas does notbypass the filter or reach the downstream side of the filter withoutbeing purified. The purifying efficiency is thus prevented from beinglowered.

If the fluid blocking member is provided at the downstream end portionof the inner case, communication between the space including thedownstream end surface of the filter and the clearance is blocked. Thisstructure further reduces the heat energy loss caused by the heattransmission from the exhaust gas to the outer case. The costs are thusfurther saved.

If the fluid blocking member is configured by the flange, the flangeserves also as a portion to which the inner case is secured. This avoidsan increase in the number of the parts and complication of thestructure. Further, the filter may be removed from the outer case in astate accommodated in the inner case. The replacement of the filter thusbecomes relatively easy, as compared to a conventional case. As aresult, maintenance is facilitated.

If a heater for regenerating the filter and heat reflector are provided,the heat of the heater is reflected by the heat reflector. The heatenergy loss of the heater is thus decreased, and the filter is heatedefficiently. Further, since the heat reflector is porous, the flow ofthe exhaust gas to the filter is not hampered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of an exhaust gaspurifying device according to a first embodiment of the presentinvention;

FIG. 2 is a cross-sectional view showing the exhaust gas purifyingdevice of FIG. 1;

FIG. 3 is an exploded perspective view showing a casing structure of theexhaust gas purifying device of FIG. 1;

FIG. 4 is an end view showing a filter employed in the exhaust gaspurifying device of FIG. 1;

FIG. 5 is a cross-sectional view showing a portion of the filter;

FIG. 6 is a cross-sectional view showing an exhaust gas purifying deviceof a second embodiment;

FIG. 7 is a cross-sectional view showing an exhaust gas purifying deviceof a third embodiment;

FIG. 8 is a cross-sectional view showing an exhaust gas purifying deviceof a fourth embodiment; and

FIG. 9 is a cross-sectional view showing an exhaust gas purifying deviceof a fifth embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

An exhaust gas purifying device 1 according to a first embodiment of thepresent invention will hereafter be described with reference to FIGS. 1to 5.

As illustrated in FIG. 1, the exhaust gas purifying device 1 purifiesexhaust gas discharged by a diesel engine 2, which serves as an internalcombustion engine. The diesel engine 2 includes a plurality ofnon-illustrated cylinders. A branch 4 of a metal exhaust manifold 3 iscoupled to each of the cylinders. The branches 4 are connected to amanifold body 5. Thus, the exhaust gas sent from each of the cylindersis concentrated at one point.

A first exhaust pipe 6 and a second exhaust pipe 7, which are formed ofmetal, are disposed at a downstream side of the exhaust manifold 3. Anupstream end of the first exhaust pipe 6 is coupled to the manifold body5. The exhaust gas purifying device 1 is located between the firstexhaust pipe 6 and the second exhaust pipe 7.

With reference to FIG. 2, the exhaust gas purifying device 1 includes acylindrical casing 21. An exhaust gas purifying filter 13 isaccommodated in the casing 21. The filter 13 will hereafter beexplained.

Since the filter 13 is used for removing diesel particulates, the filter13 is called also as a diesel particulate filter (DPF). The filter 13 ofthe first embodiment, as shown in FIGS. 4 and 5, is formed by bundling aplurality of sintered honeycomb bodies F as one body. The sinteredhoneycomb bodies F located in the middle portion of the filter 13 areshaped as square rods. A plurality of sintered honeycomb bodies F thatare shaped differently, or unlike the square rods, are arranged aroundthe honeycomb bodies F that are shaped as the square rods. Accordingly,the filter 13 as a whole is shaped as a cylinder.

In the first embodiment, each of the sintered honeycomb bodies F isformed from a sintered body of porous silicon carbide (SiC), which is atype of sintered ceramic body. However, other than the sintered body ofsilicon carbide, a sintered body of silicon nitride or sialon or aluminaor cordierite may be selected. A plurality of through holes 11, each ofwhich has a substantially square cross-sectional shape, are formed ineach of the sintered honeycomb bodies F and are arranged regularly in anaxial direction. The through holes 11 are spaced from one another bycell walls 12. An opening of each through hole 11 is sealed by a plug 14(in this embodiment, a sintered body of porous silicon carbide) at oneof end surfaces 13 a, 13 b. Each of the end surfaces 13 a, 13 b as awhole thus presents a diced pattern. As a result, a number of cells eachhaving a square cross-sectional shape are formed in each sinteredhoneycomb body F. Approximately half of the cells open at the upstreamend surface 13 a, while the remaining cells open at the downstream endsurface 13 b.

As shown in FIGS. 4 and 5, the sintered honeycomb bodies F are boundtogether by an adhesive 15 at the outer peripheral surfaces. Theadhesive 15 serves to compensate thermal expansion of the sinteredhoneycomb bodies F. In other words, the adhesive 15 prevents cracks frombeing caused by thermal stress. In the first embodiment, a heatresistant ceramic adhesive in which ceramic fibers are dispersed is usedas the adhesive 15. It is preferred that silicon carbide powders, inaddition to the ceramic fibers, are dispersed in the adhesive 15.

With reference to FIGS. 2 and 3, the casing 21, which accommodates thefilter 13, is configured by a plurality of metal tubular members. Morespecifically, in the first embodiment, three members including a heatercase 22, an inner case 23, and an outer case 24 (all formed of SUS304)configure the casing 21. The heater case 22, serving as a heating meansaccommodating case, forms a portion of a heater unit. The inner case 23forms a portion of a filter unit.

The heater case 22 defines a sealed space for preventing exhaust gasfrom entering the heater case 22. The heater case 22 thus provides aheat insulating effect. A mat-shaped heat insulating material 8, themain component of which is ceramic fibers, is packed in the space of theheater case 22. This improves the heat insulating effect.

Each of the inner case 23 and the outer case 24 has a cylindrical shape.The inner case 23, which is accommodated in the outer case 22, issmaller than the outer case 24 (the longitudinal dimension and thediameter of the inner case 23 are smaller than those of the outer case24). Therefore, when the cases are assembled, a clearance C1 having aconstant dimension is defined between the outer peripheral surface ofthe inner case 23 and the inner peripheral surface of the outer case 24,which is arranged around the inner case 23. In the first embodiment, thedimension of the clearance C1 is approximately 1 to 5 millimeters. Aflange 25 of the inner case 23 serves also as a fluid blocking memberthat blocks communication between the space including the upstream endsurface 13 a of the filter 13 and the clearance C1.

The flange 25 projects from the outer peripheral surface of an upstreamend portion of the inner case 23. In the same manner, a flange 26projects from the outer peripheral surface of an upstream end portion ofthe outer case 24. The flanges 25, 26 are designed to define equal outerdiameters. A plurality of bolt holes 28 are formed in the flange 25 andare spaced from adjacent ones at certain intervals. In the same manner,a plurality of bolt holes 29 are formed in the flange 26 and are spacedfrom adjacent ones at certain intervals. Attachment bolts 27 are passedthrough the bolt holes 28, 29. The bolt holes 29 of the flange 25 arelocated at positions corresponding to the bolt holes 28 of the flange26. In the same manner, a plurality of bolt holes 31 are formed in adownstream end surface of the heater case 22 at positions correspondingto the bolt holes 28, 29. Thus, if the bolts 27 are passed through thecorresponding bolt holes 28, 29, 31 and fastened as such, the cases 22,23, 24 are fixed to each other to form one body, or the casing 21. Ifthe bolts 27 are removed from the bolt holes 28, 29, 31, the casing 21is separated into three parts. In other words, it may be understood thatthe flange 25 of the inner case 23 is secured to the flange 26 of theouter case 24 in an attachable or detachable manner.

The mat-shaped heat insulating material 17 containing ceramic fibers iswrapped around the filter 13. In this state, the filter 13 is held inthe inner case 23. That is, the inner case 23 supports the outerperipheral surface of the filter 13 through the heat insulating material17. A filter support 30 projects from the inner peripheral surface ofthe downstream end of the inner case 23 and extends along the entirecircumference. The filter support 30 is abutted by an outer peripheralportion of the downstream end surface 13 b of the filter 13. Thisstructure prevents the filter 13 from falling from the inner case 23 tothe downstream side.

A support piece 30 a is attached to the inner peripheral surface of theupstream end portion of the inner case 23 and extends along the entirecircumference. The support piece 30 a is abutted by an outer peripheralportion of the upstream end surface 13 a of the filter 13. Thisstructure prevents the filter 13 from falling from the inner case 23 tothe upstream side. Further, the heat insulating material 17 is preventedfrom being displaced. A plurality of support pieces 30 a may be deployedas spaced from adjacent ones at predetermined angular intervals.

A coupling portion 32 projects from a middle portion of the, upstreamend surface of the heater case 22 and is coupled with a downstream endof the first exhaust pipe 6. Further, a coupling portion 33 projectsfrom a middle portion of the downstream end surface of the outer case 24and is coupled with an upstream end of the second exhaust pipe 7.

As illustrated in FIGS. 1 and 2, the heater case 22, a component of thecasing 21, accommodates a heater 34 and a temperature detector 35. Theheater 34 serves as an electric heating means for regenerating thefilter and is located at an upstream position with respect to the filter13. In the first embodiment, an AC heater is used as the heater 34. Theheater 34 is formed by winding a cable spirally. More specifically, thecable includes a conductive core of a nichrome wire covered by a sheathof magnesia, which presents improved insulating performance.

The heater 34 is opposed to the upstream end surface 13 a of the filter13 and is spaced from the upstream end surface 13 a at a certaininterval. Two end portions 34 a of the heater 34 extend through an outerperipheral portion of the heater case 22 to the exterior of the casing21. The core projecting from each of the end portions 34 a of the heater34 is electrically connected to a connector through a glass tube. Theconnector, as shown in FIG. 1, is electrically connected to a drivercircuit of a control unit U1 controlling the regenerating operation ofthe filter 13. Thus, when necessary, the control unit U1 operates tosupply power to the heater 34 from an external power source B1. Thisenables the heater 34 to generate heat from the entire portion such thatthe temperature rises to 800 to 900 degrees Celsius.

With reference to FIG. 2, a punching plate 38 serving as a porous heatreflector is disposed at an upstream position with respect to the heater34. The punching plate 38 is a disk-shaped plate member and is formed ofstainless steel (SUS304) in the first embodiment. The outer periphery ofthe punching plate 38 is bonded with the inner peripheral surface of theheater case 22 through, for example, welding. The punching plate 38 isthus secured to the heater case 22. Further, the heater 34 is fixed tothe punching plate 38 by a fixing tool 37.

A number of through holes 38 a extend through the punching plate 38. Thethrough holes 38 a are arranged regularly to cover substantially theentire area of the punching plate 38. Therefore, after being dischargedfrom the first exhaust pipe 6, exhaust gas passes through the throughholes 38 a to reach the filter 13. Further, the punching plate 38reflects the heat generated by the heater 34, preventing the heat frombeing released to the exterior. In other words, the heat reflected bythe punching plate 38 is supplied to the filter 13 as radiation heat.The filter 13 is thus efficiently heated.

A ceramic foam reflector 39 serving as another porous heat reflector isdisposed at the downstream side of the filter 13, as illustrated in FIG.2. In the first embodiment, the ceramic foam reflector 39 is supportedby a heat reflector support 40, which projects from the inner peripheralsurface of the outer case 24 and extends along the entire circumference.The ceramic foam reflector 39 is a porous body formed of aluminumnitride or the like and presents fluid permeability and heat insulatingproperties. Therefore, like the upstream side of the filter 13, a heatinsulating effect is ensured at the downstream side of the filter 13.

With reference to FIG. 2, the temperature detector 35 is deployed in thevicinity of the heater 34. The temperature detector 35 is a rod-shapedbody, and a temperature detecting portion 35 a is formed at a distal endof the temperature detector 35. The temperature detector 35 of thisembodiment includes a sheath thermocouple covered by a protecting pipeformed of stainless steel or the like. The temperature detecting portion35 a is exposed from a distal end of the covered sheath thermocouple.The temperature detecting portion 35 a is placed in the space defined bythe cable spirally wound in the heater 34. The temperature detector 35is electrically connected to the control unit U1.

The operation of the exhaust gas purifying device 1 configured asdescribed above will hereafter be explained.

The casing 21 accommodating the filter 13 is deployed in the exhaust gaspassage, or between the first exhaust pipe 6 and the second exhaust pipe7. In this state, if the engine 2 is started, exhaust gas is sent to theinterior of the casing 21. That is, after being discharged from thefirst exhaust pipe 6, the exhaust gas flows first into the cells openingat the upstream end surface 13 a of the filter 13. The gas thenpermeates the cell walls 12 and reaches the adjacent cells, which openat the downstream end surface 13 b of the filter 13. Further, throughthe openings of the cells, the exhaust gas is discharged from thedownstream end surface 13 b of the filter 13.

However, the soot contained in the exhaust gas is not permitted topermeate the cell walls 12 and is trapped in the cells. As a result, theexhaust gas is purified before being discharged from the downstream endsurface 13 b of the filter 13. The purified gas further flows in thesecond exhaust pipe 7 and is eventually released to the atmosphere.Afterwards, the heater 34 is powered to heat the filter 13 andsupporting air is supplied such that the soot is burned and removed.More specifically, the soot in the vicinity of the upstream end surface13 a of the filter 13 starts to burn. Burning of the soot is graduallyspread to the downstream end surface 13 b. By maintaining the sootburning for a certain time period, the filter 13 is regenerated.

Accordingly, the first embodiment has the following effects.

(1) The casing 21 of the first embodiment has a double structure formedby the inner case 23 and the outer case 24. The inner case 23 supportsthe outer peripheral surface of the filter 13. The outer case 24 isarranged around the inner case 23. Further, the cases 23, 24 are spacedfrom each other at an interval corresponding to the clearance C1.

Although the outer peripheral surface of the filter 13 is held incontact with the inner case 23, the clearance C1 is ensured between theinner case 23 and the outer case 24. In other words, a heat insulatingair layer is formed between the cases 23, 24. The heat insulting airlayer presents relatively low heat conductivity as compared to metal andhampers the heat transmission from the inner case 23 to the outer case24. Further, air convection does not occur readily in the clearance C1.This structure stops heat from escaping to the exterior of the filter13. The temperature of the filter 13 is thus efficiently raised. Inother words, the exhaust gas purifying device 1 has decreased energyloss and thus saves costs. In addition, since the heat energy applied tothe filter 13 is also decreased, the power supplied to the heater 34 isreduced and the time required for regenerating the filter 13 isshortened.

(2) The casing structure of the first embodiment prevents heat fromescaping from the outer peripheral portion of the filter 13. Thus, atemperature difference is hardly caused between the middle portion ofthe filter 13 and the outer peripheral portion of the filter 13. Thisavoids generation of relatively large thermal stress, which leads tocracks damaging the filter 13. Therefore, the exhaust gas purifyingdevice 1 presents improved strength and has relatively high reliability.

(3) Since the casing structure of the first embodiment stops heat fromescaping from the outer peripheral portion of the filter 13, thetemperature of the outer peripheral surface of the outer case 24 isreliably lowered. Therefore, the components attached to the outersurface of the outer case 24 and those disposed around the outer case24, for example, do not necessarily have to have high heat resistingperformance, as compared to conventional counterparts.

(4) In the casing structure of the first embodiment, the flange 25serving as the fluid blocking member, which is formed at the upstreamend portion of the inner case 23, blocks communication between the spaceincluding the upstream end surface of the filter 13 and the clearanceC1. Thus, relatively hot exhaust gas does not flow into the clearance C1in which the heat insulating air layer is located. This prevents theheat from the exhaust gas from being transmitted to the outer case 24,reducing the heat energy loss otherwise caused by such heattransmission. The costs are thus further saved. In addition, the exhaustgas does not bypass the filter 13 or reach the downstream side withoutbeing purified. This structure prevents the purifying efficiency frombeing lowered.

(5) In the casing structure of the first embodiment, the flange 25serving as the fluid blocking member also functions as a portion towhich the inner case 23 is fixed. This structure avoids an increase inthe number of the parts and complication of the configuration.

Further, this structure allows the filter 13 to be removed from theouter case 24 in a state accommodated in the inner case 23. It is thuspossible to easily replace the filter 13, as compared to a conventionalcase. Also, a major portion of the downstream end surface 13 b of thefilter 13 is exposed from the inner case 23. This makes it relativelyeasy to clean ashes after the filter 13 is removed from the casing 21.In this manner, the casing structure of this embodiment facilitates themaintenance.

(6) In the casing structure of the first embodiment, the punching plate38 serving as the heat reflector reflects the heat of the heater 34.Further, the heat of the filter 13, which is heated by the heater 34, isreflected by the ceramic foam reflector 39 also serving as the heatreflector. The heat energy loss of the heater 34 is thus decreased suchthat the filter 13 is efficiently heated. This structure contributes toshortening of the time required for regenerating the filter 13. Inaddition, since the heat reflectors are porous, the exhaust gas flow toand from the filter 13 is not hampered.

(7) In the first embodiment, the outer peripheral portion of theupstream end surface 13 a of the filter 13 is held in contact with thefilter support piece 30 a formed at the upstream end portion of theinner case 23. This structure prevents the heat insulating material 17from being corroded. Further, when the filter 13 is removed from thecasing 21 and is washed in water for cleaning ashes, the heat insulatingmaterial 17 is stopped from being displaced.

Other embodiments of the present invention will hereafter be described.

In a second embodiment illustrated in FIG. 6, a flange 41 projects fromthe outer peripheral surface of the downstream end portion of the innercase 23 and extends along the entire circumference. In this embodiment,the flange 41 serving as a fluid blocking member blocks communicationbetween the space including the downstream end surface 13 b of thefilter 13 and the clearance C1. This structure further reduces the heatenergy loss caused by the heat transmission from the exhaust gas to theouter case 24, as compared to the first embodiment. This effect isbrought about by the fact that a heat insulating air layer morepreferable than that of the first embodiment is formed in the clearanceC1. Therefore, costs are further saved. In addition, the filter 13 isfurther securely fixed as long as the outer periphery of the flange 41is held in contact with the inner peripheral surface of the outer case24, as shown in FIG. 6.

Further, it is desirable that a clearance is defined between the flange41 and the inner peripheral surface of the outer case 24. The clearancemakes it possible to remove the inner case 23 smoothly from the outercase 24, even if the dimensions of the inner case 23 are changed due tothermal expansion.

In a third embodiment illustrated in FIG. 7, the casing 21 includes anadditional outer case 42, other than the outer case 24. The casing 21thus has a triple structure. The additional outer case 42 is arrangedbetween the outer case 24 and the inner case 23. This structure has twoclearances C1, each of which serves as a heat insulating air layer.Further, a flange 43 projects from the outer peripheral surface of theupstream end portion of the outer case 42. The flange 43 is deployed asclamped between the flanges 25, 26.

In a fourth embodiment illustrated in FIG. 8, a flange 41 a serving as afluid blocking member projects from the inner peripheral surface of theouter case 24. The flange 41 a is engaged with the filter support 30 ofthe inner case 23. The communication between the space including thedownstream end surface 13 b of the filter 13 and the clearance C1 isblocked by the flange 41 a. This structure further reduces the heatenergy loss caused by the heat transmission from the exhaust gas to theouter case 24, as compared to a comparative example (which will bedescribed later).

Also, the fourth embodiment is not provided with the punching plate 38such that the configuration becomes simple.

In a fifth embodiment illustrated in FIG. 9, the support piece 30 a,which is otherwise formed at the upstream end portion of the inner case23, is not provided, as is clear from comparison with the firstembodiment. The fifth embodiment thus has the operational effects of thefirst embodiment, except for that of the support piece 30 a.

The exhaust gas purifying devices of the illustrated embodiments weresubjected to an operational test. One cycle of the test was defined bysending exhaust gas to each of the devices, heating the filter by theheater, and supplying supporting air for burning the soot. The testincluded ten cycles. The test results are shown in Table 1. Thelongitudinal dimension of the filter 13 was 150 millimeters. TABLE 1 Up-Down- Temperature Insulator stream stream Support Punching DifferenceCorrosion Embodiment Drawing Blocker Blocker Structure Piece Plate (C.°) Crack Size 1 Formed None Double Formed Formed 30 None 0 2 FormedFormed Double Formed Formed 20 None 0 3 Formed None Triple Formed Formed26 None 0 4 Formed Formed Double Formed None 40 None 0 5 Formed NoneDouble None Formed 30 None 15 Comparative — None None Single None None150 Detected 15

In Table 1, the temperature difference indicates the temperaturedifference between the middle portion and the outer peripheral portionof the filter 13. The insulator corrosion size indicates the size of theportion of the heat insulating material lost due to corrosion in thetest. Detection of cracks was conducted visually after the test wascompleted. A purifying device prepared as the comparative example didnot include any of the inner case, the upstream blocking member, thedownstream blocking member, the support piece, and the punching plate.This purifying device was also subjected to the operational test.

As indicated clearly in Table 1, in the comparative example, thetemperature difference between the middle portion and the outerperipheral portion of the filter was enlarged, causing cracks. Incontrast, the temperature difference was relatively small in the firstto fifth embodiments. Therefore, no crack was detected, and thedurability was improved.

Further, as is clear from comparison among the comparative example, thefirst embodiment, and the fifth embodiment, the support piece 30 aoperated efficiently for preventing the heat insulating material frombeing corroded.

The present invention is not restricted to the illustrated embodimentsbut may be embodied in the following forms.

In the illustrated embodiments, the punching plate 38 and the ceramicfoam reflector 39 are employed as the porous heat reflectors. However,the heat reflectors are not limited to those of the embodiments but maybe, for example, a body formed of metal meshes or ceramic fibers.

The ceramic foam reflector 39, which is provided at the downstream sideof the filter 13, may be replaced by the punching plate 38. Further, theceramic foam reflector 39 may be omitted. In this manner, the number ofthe components of the exhaust gas purifying device 1 is decreased.

The heater 34 does not necessarily have to be an AC heater but may be,for example, a DC heater. Further, the electric heating means such asthe electric heater may be replaced by a non-electric heating means suchas a burner.

The fluid blocking member does not necessarily have to be the flange 25,which projects from the outer peripheral surface of the end of the innercase 23. The fluid blocking member may be a different structure providedseparately from the inner case 23.

The flange 25 may be fixed to the outer case 24, which is the outermostlayer, through, for example, welding, such that the flange 25 isprohibited from being attached to or detached from the outer case 24.

1. An exhaust gas purifying device for use with an engine having anexhaust passage, the exhaust gas purifying device comprising: a tubularcasing disposed in the exhaust passage of the engine and a filteraccommodated in the casing for collecting, burning, and removingparticulates contained in the exhaust gas discharged by the engine, thecasing having a multiple case structure including an inner casesupporting an outer peripheral surface of the filter and at least oneouter case arranged around the inner case, with the inner and outercases being spaced from each other with and a clearance defined betweenthe inner and outer cases.
 2. An exhaust gas purifying device for usewith an engine having an exhaust passage, the exhaust as purifyingdevice comprising: a tubular casing disposed in the exhaust passage ofthe engine and a filter accommodated in the casing for collecting,burning, and removing particulates contained in the exhaust gasdischarged by the engine, the casing having a double case structureincluding an inner case supporting an outer peripheral surface of thefilter and an outer case arranged around the inner case, with the innerand outer cases being spaced from each other and a clearance definedbetween the inner and outer cases.
 3. The exhaust gas purifying deviceaccording to claim 1, further comprising a fluid blocking member formedat an upstream end portion of the inner case for blocking communicationbetween a space including an upstream end surface of the filter and theclearance.
 4. The exhaust gas purifying device according to claim 1,further comprising a support piece provided at an upstream side of theinner case.
 5. The exhaust gas purifying device according to claim 3,further comprising a fluid blocking member provided at a downstream sideof the inner case and between the inner case and the outer case forblocking communication between a space including a downstream endsurface of the filter and the clearance.
 6. The exhaust gas purifyingdevice according to claim 3, wherein the fluid blocking member is aflange projecting from an outer peripheral surface of an end of theinner case, and the flange is secured to an outermost component of theouter case in an attachable or detachable manner.
 7. The exhaust gaspurifying device according to claim 1, further comprising a heater forregenerating the filter disposed at an upstream position with respect tothe filter, and a porous heat reflector arranged at a further upstreamposition with respect to the heater.
 8. A casing structure for anexhaust gas purifying device having a filter for use with an engine, thecasing structure comprising: a multiple case structure including aninner case supporting an outer peripheral surface of the filter forcollecting, burning, and removing particulates contained in the exhaustgas discharged by the engine and at least one outer case arranged aroundthe inner case, the casing structure including inner and outer casesspaced from each other with a clearance defined between the inner andouter cases.
 9. The exhaust gas purifying device according to claim 1,wherein the filter is a ceramic filter.
 10. The exhaust gas purifyingdevice according to claim 1, wherein the filter is a sintered honeycombbody formed of porous silicon carbide.
 11. The exhaust gas purifyingdevice according to claim 1, wherein the filter is an integral bodyformed by bundling a plurality of prism-shaped sintered honeycomb bodiesformed of porous silicon carbide and binding the bundled bodies with anadhesive agent.
 12. A casing structure for an exhaust gas purifyingdevice having a filter for use with an engine the casing structurecomprising: a multiple case structure including an inner case supportingan outer peripheral surface of the filter for collecting, burning, andremoving particulates contained in the exhaust gas discharged by anengine, at least one outer case arranged around the inner case, and aheating means accommodating case accommodating a heating means forregenerating the filter arranged at an upstream side of the outer case,with the inner and outer cases spaced from each other and a clearancedefined between the inner and outer cases.
 13. The exhaust gas purifyingdevice according to claim 2, further comprising a fluid blocking memberformed at an upstream end portion of the inner case for blockingcommunication between a space including an upstream end surface of thefilter and the clearance.
 14. The exhaust gas purifying device accordingto claim 2, further comprising a support piece is provided at anupstream side of the inner case.
 15. The exhaust gas purifying deviceaccording to claim 3, further comprising a support piece is provided atan upstream side of the inner case.
 16. The exhaust gas purifying deviceaccording to claim 3, further comprising a fluid blocking memberprovided at a downstream side of the inner case and between the innercase and the outer case for blocking communication between a spaceincluding a downstream end surface of the filter and the clearance. 17.The exhaust gas purifying device according to claim 16, wherein thefluid blocking member is a flange projecting from an outer peripheralsurface of an end of the inner case, and the flange is secured to anoutermost component of the outer case in an attachable or detachablemanner.
 18. The exhaust gas purifying device according to claim 5,wherein the fluid blocking member is a flange projecting from an outerperipheral surface of an end of the inner case, and the flange issecured to an outermost component of the outer case in an attachable ordetachable manner.
 19. The exhaust gas purifying device according toclaim 2, further comprising a heater for regenerating the filterdisposed at an upstream position with respect to the filter, and aporous heat reflector arranged at a further upstream position withrespect to the heater.
 20. The exhaust gas purifying device according toclaim 3, further comprising a heater for regenerating the filterdisposed at an upstream position with respect to the filter, and aporous heat reflector arranged at a further upstream position withrespect to the heater.